The objectives of the present proposal are to pursue the discovery that prolactin (PRL) increases the transport of taurocholate (TC), a major bile acid, across the hepatocyte. TC transport is decreased in pregnancy when PRL levels are low, but rebounds post partum when PRL levels are high. The rebound post partum can be prevented by blockade of PRL secretion, and treatment of rats with ovine PRL (oPRL) increases the transport of TC in a dose-dependent manner. The general objectives are to characterize further the changes in transport seen post partum and following oPRL treatment and to identify the mechanism(s) by which PRL increases TC transport. The specific aims are to 1) characterize the time course of the changes in TC transport throughout lactation, 2) determine if transport of other substrates, specifically nobile acid organic anions, is increased post partum an by treatment of rats wit oPRL, 3) test the hypothesis that PRL increases TC transport by increasing the hepatocytes membrane potential, 4) test the hypothesis the PRL increases Na/TC cotransport by increasing synthesis of messenger RNA and synthesis of transport protein, 5)test the hypothesis that PRL increases TC transport across the canalicular membrane by increasing synthesis of messenger RNA and synthesis of the ecto-ATPase/bile acid transporter protein, and 6) characterize Na+/TC cotransport in basolateral plasma membrane vesicles and potential dependent and ATP- dependent transport of TC in canalicular membrane vesicles and potential- dependent and ATP-dependent transport of TC in canalicular membrane vesicle in controls s oPRL-treated rats. Approaches used to answer these questions include measurements of the secretory rate maximum for transport in isolated hepatocytes, characterization of transport in basolateral and canalicular membrane vesicles, quantitation of changes in messenger RNA using cDNA probes to the Na+/TC cotransporter and the ecto-ATPase/bile acid transporter, quantitate rates of nuclear transcription of message of these genes, and use of specific antibodies to quantitate transport proteins. The finding that PRL increases TC transport capacity in a mammalian species suggests the possibility of similar functions in humans, particularly in women. Characterization of the mechanisms by which PRL increases TC transport is essential for understanding the normal physiology and regulation of these studies is therefore to understand the mechanisms by which PRL regulates TC transport and bile secretory function in order to develop effective therapeutic interventions in cholestatic liver disease.